Electrostatic coating systems in which an electrical charge is placed on a workpiece during the coating process are well known. Generally, in such systems, the workpiece is supported on or from a moving conveyor and the electrical charge is transferred to the workpiece from a charge collector mounted for compliant movement with the workpiece. The charge collector may be charged directly by physically contacting a source of electrical potential or inductively charged by being positioned in close proximity to an electrically charged conductor. Examples of the direct contact charging arrangements are shown in U.S. Pat. No. 3,113,037 issued Dec. 3, 1963 to Tamotsu Watanabe and U.S. Pat. No. 3,937,180 issued Feb. 10, 1976 to Richard F. Wiggins. A system having an arrangement for inductively charging a workpiece is shown in U.S. Pat. No. 4,158,344 issued to Thomas L. Bagby, et al and assigned to the assignee of the present invention.
A problem common to all charged-workpiece electrostatic coating systems is dissipation, or removal, of the charge after completion of the coating operation. Watanabe solves the problem by pressing a conductive support rod carrying the charged workpiece against a spring-loaded switch. The switch controls energization of an operating coil in a remotely positioned oil immersed second switch, and thereby completes a conductive path to ground to remove the charge on the support rod and workpiece.
Wiggins addresses the problem of workpiece charge dissipation by sequentially physically contacting a series of spring-loaded buttons each connected to a resistor, the resistors respectively having progressively lower resistance to ground. As the charged workpiece exits the spray zone, a shoe attached to a hanger supporting a workpiece, sequentially contacts the spring-loaded buttons and the charge on the workpiece is incrementally and progressively reduced.
In yet another approach to the problem, Bagby, et al incorporate a bleeder resistor in the workpiece support hanger. The resistor is positioned between a charge collector and a grounded conveyor system thereby allowing the charge on the workpiece and collector to drain to ground shortly after leaving the charging zone. The Bagby, et al arrangement avoids the mechanical switching and multiple contacts required in the earlier Watanabe and Wiggins systems. However, Bagby's positioning of a resistor in the workpiece hanger requires increased hanger length; a limitation that may be undesirable in systems having only limited clearance between the workpiece and conveyor.
The present invention is directed to overcoming the problems set forth above. In particular, a charge removing apparatus constructed according to the present invention eliminates the need for multiple switches and resistors and provides a simplified non-arcing arrangement for dissipating the electrical charge on a workpiece in a controlled manner.